Ground Fault Interrupter (GFI) circuits are used in applications where a potential electric shock hazard exists, and are designed to interrupt the electrical load current should an individual inadvertently contact an exposed conductor. The sensing portion of the GFI circuit operates as shown in FIG. 1. Two primary windings on the GFI transformer are phased such that, H when the load current (I.sub.1) and the return current (I.sub.2) are equal, the two fluxes cancel, and the induced current in the transformer secondary fed to the threshold detector is essentially zero.
Should someone touch an exposed portion of the load circuit and complete a path to ground, as shown in FIG. 2, the load current will be greater than the return current by an amount equal to the shock current (I.sub.3) through the victim. Expressed mathematically, I.sub.1 =I.sub.2 +I.sub.3. Under this condition, I.sub.1 and I.sub.2 are no longer equal, and their fluxes will not cancel. A current, proportional to I.sub.3, will be induced into the secondary winding of the GFI transformer. This induced current, I.sub.3, may then be compared to a predetermined threshold level, and, if determined to be excessive, can initiate an immediate removal of power by opening a relay, contactor, or semiconductor switch.
The ground fault detection circuit described above operates well for most applications, and has been in widespread use for many years. A problem has been discovered, however, when a physically large resistive load is installed very close to a large grounded metal surface. One example of this type of installation is an electrothermal de-icing system bonded to the wing of an aircraft where the size of the resistive heating element is almost the same size as the bonding surface. A resistive heater structure for use in an electrothermal de-icing system is described in U.S. Pat. No. 4,942,078 issued to Newman et al., incorporated herein by reference. Newman et al. teach a plurality of layers of structural fabric which have been treated and prepared with a laminating resin and cured into a laminate structure. At least one of the layers of fabric is rendered conductive by being treated with conductive polymer. The use of non-woven, nickel-plated carbon fiber cloth is another alternative material for use in an electrothermal de-icing system. A typical non-woven web is described in U.S. Pat. No. 4,534,886 issued to Kraus et al., incorporated herein by reference. Kraus et al. teach an electrically conductive non-woven web which contains both conductive fibers and conductive particles. In a common embodiment, the non-woven heater element would be encapsulated between layers of an adhesive impregnated cloth. The most common cloth would be fiberglass, either woven or non-woven. A commercially available conductive fiber for use as an electrothermal de-icing system is manufactured by Technical Fibre Products, Ltd. located in Kendal England.
In an electrothermal de-icing system assembly, the resistive heater element and the wing form the two plates of a parallel plate capacitor, and the insulation between the plates acts as the dielectric as shown schematically in FIG. 3. Once the electrothermal system is installed, the dielectric strength of the insulator may be very good, and the resistive leakage current from the heater to the wing may be negligible. However, a small but significant alternating current can flow through the unwanted capacitor to the grounded surface. This capacitive leakage current, I.sub.4, can unnecessarily trigger a ground fault interrupter installed on the system, even when no shock current, I.sub.3, is present. Simply raising the threshold level in the GFI is not a practical solution to the problem because the capacitance of the unwanted capacitor is not well controlled and is, therefore, not predictable. Furthermore, the safety of maintenance workers is of prime importance, and any suggestion of compromising the safety of the workers may raise serious safety concerns.
Generally, other possible solutions utilizing alternate current sensing devices are not viable in a de-icing application. The capacitance between the heater assembly and the wing is not adequately predictable because it is a function of the adhesive thickness and temperature. The adhesive thickness varies with the installation, and the capacitance can change up to 20% over the heater's normal temperature range.
It is therefore an object of the invention to provide a safe and reliable ground fault interrupter circuit for electrothermal de-icing applications.
It is a further object of the present invention to provide a reliable ground fault protection device that works independently of the capacitance between the heater and the wing.